Source: https://guthlabs.ai/docs/instruments/household-water-usage-calculator/mechanism/

instrument mechanism / verified-clean

# Household Water Usage Calculator mechanism

The inspectable method record for Household Water Usage Calculator, kept separate from implementation and qualification.

Status: **VERIFIED_CLEAN**Lifecycle: **verified-clean**

## Model scope

The arithmetic engine is standard and produces valid estimates under the stated assumptions. The instrument needs explicit units for every fixture-table value, an explicit frequency normalization rule, a defined month convention, and a documented table source for each fixture category. The honest label should state that results are estimates based on rated or typical fixture performance and user-entered frequency; actual use varies with fixture age, efficiency, water pressure, operating behavior, leaks, and billing structure.

- The core volume formula is correct only when each fixture rate and its usage measure have compatible units: flow rate must be multiplied by time, while per-use quantities such as gallons per flush or gallons per load must be multiplied by counts.

- The specification does not define how daily, weekly, and monthly frequencies are normalized, nor whether a month means 30 days, calendar-month days, or an average month.

- The fixture table is described as EPA WaterSense, but WaterSense product standards do not provide one universal typical-flow table for every listed fixture category; the source and applicability of each fixture value must be defined.

- The cost formula is correct for a flat volumetric rate in dollars per gallon, but it is incomplete for common tiered rates, fixed service charges, sewer charges, taxes, or minimum bills.

## Units and conventions

- Shower and faucet rates are in US gallons per minute (gpm).

- Toilet consumption is in US gallons per flush (gpf), so it is multiplied by flush count rather than duration.

- All usage frequencies in the daily and monthly examples are occurrences per day; monthly output uses exactly 30 days.

- Water rate is a flat US dollars per US gallon and excludes fixed charges, sewer fees, taxes, and tiered pricing.

- A duration supplied in seconds must be converted to minutes before multiplying by a gpm rate.

- The table values are nominal rated or standardized values; actual fixture flow can vary with pressure, temperature, fixture age, and operating condition.

## Inspectable vector

One of 4 source-bound review vectors:

````
{
  "inputs": {
    "period": "daily",
    "monthly_days": 30,
    "shower": {
      "flow_rate_gpm": 2,
      "duration_min_per_use": 8,
      "uses_per_day": 1
    },
    "toilet": {
      "gallons_per_flush": 1.28,
      "flushes_per_day": 5
    },
    "water_rate_usd_per_gallon": 0.008
  },
  "expected_outputs": {
    "daily_gallons": 22.4,
    "monthly_gallons": 672,
    "daily_cost_usd": 0.1792,
    "monthly_cost_usd": 5.376
  },
  "reference_data": {
    "shower_water_rate_gpm": 2,
    "shower_source_convention": "EPA WaterSense specification maximum labeled showerhead flow rate, 2.0 gpm at 80 psi",
    "toilet_water_use_gpf": 1.28,
    "toilet_source_convention": "EPA WaterSense high-efficiency toilet maximum effective flush volume, 1.28 gpf"
  },
  "working": "Shower: 2.0 gpm × 8 min/use × 1 use/day = 16.0 gal/day. Toilet: 1.28 gal/flush × 5 flushes/day = 6.40 gal/day. Total daily use = 16.0 + 6.40 = 22.40 gal/day. Monthly use = 22.40 × 30 = 672.0 gal. Daily cost = 22.40 × $0.008 = $0.1792. Monthly cost = 672.0 × $0.008 = $5.376."
}
````

## Failure boundary

- Division by zero or invalid results can occur if the implementation converts a billing rate supplied per 1,000 gallons by dividing by a zero unit multiplier; validate all unit-conversion denominators.

- Negative flow rates, durations, counts, frequencies, or prices should be rejected.

- A zero flow rate may represent an invalid missing table value and should not be silently treated as zero usage.

- Do not multiply a gallons-per-flush or gallons-per-load value by duration; those quantities require counts.

- Monthly totals can be wrong if the implementation silently assumes 30 days for a calendar month.

- A single flat rate can materially misstate cost under tiered utility billing or when sewer and fixed charges are included.

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/household-water-usage-calculator/>)[03**Implementation**](<https://guthlabs.ai/docs/instruments/household-water-usage-calculator/implementation/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/household-water-usage-calculator/qualification/>)

## Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

- `VERIFIED_LEDGER.json`

- `evidence-tranche7/tranche7-results.json`